feat: MERGE Statement Support (SQL:2003)

[productdevbook]

Summary

Add MERGE statement support for upsert operations.

SQL:2003 Standard MERGE

MERGE INTO target USING source ON condition
WHEN MATCHED THEN UPDATE SET ...
WHEN NOT MATCHED THEN INSERT (cols) VALUES (vals)

Dialect Support

• MSSQL: Full MERGE support
• PostgreSQL: No MERGE (use INSERT ... ON CONFLICT)
• MySQL: No MERGE (use INSERT ... ON DUPLICATE KEY UPDATE)
• SQLite: No MERGE (use INSERT ... ON CONFLICT)

Design

• Add MergeNode to AST
• Printer generates MERGE for MSSQL, ON CONFLICT for PG/SQLite, ON DUPLICATE KEY for MySQL
• Single builder API that abstracts dialect differences

Action Items

• Add MergeNode to AST
• Implement MergeBuilder with when/matched/notMatched API
• Generate dialect-appropriate SQL
• Test across all dialects

[productdevbook]

Completed in 0650a9c. MERGE statement (SQL:2003) implemented:

• MergeNode AST with MergeWhenMatched (UPDATE/DELETE) and MergeWhenNotMatched (INSERT)
• MergeBuilder untyped builder (src/builder/merge.ts)
• TypedMergeBuilder<DB, Target, Source> with typed proxy-based ON condition
• db.mergeInto() on Sumak class
• .whenMatchedThenUpdate(), .whenMatchedThenDelete(), .whenNotMatchedThenInsert()
• Conditional WHEN clauses supported
• Printer support in BasePrinter (works across all 4 dialects)
• Tests in test/builder/merge.test.ts and test/builder/typed-merge.test.ts